Triangulum Australe, constellation in the southern sky at about 16 hours right ascension and 65° south in declination. Its brightest star is Alpha Trianguli Australis (sometimes abbreviated as Atria), with a magnitude of 1.9. The brightest stars in the constellation form an obvious triangle. This constellation was invented by Pieter Dircksz Keyser, a navigator who joined the first Dutch expedition to the East Indies in 1595 and who added 12 new constellations in the southern skies.